1. Field of the Invention
The present invention relates to processes and equipment for manufacturing a photovoltaic device; and more particularly to apparatus and methods for aligning a component of the manufacturing equipment with a wafer of material on which the photovoltaic device is being fabricated.
2. Description of the Related Art
Photovoltaic devices, such as solar cells are fabricated on a wafer of silicon or other material. Various sections of the wafer are electrically interconnected by a pattern of silver or other conductive material deposited on a surface of the wafer. Typically, a screen printing process, similar to silk screen printing element, is employed to deposit the silver in the proper pattern on the wafer surface. The printing screen used in that deposition process must be precisely aligned with the wafer in order that the silver conductive pattern is properly registered with the semiconductor devices.
Machine vision systems are commonly used in semiconductor device processing. In such systems, a video camera produces an image of the semiconductor wafer and that image is then analyzed for various purposes, such as defect detection. In other situations, the wafer image is utilized to determine whether the wafer is properly positioned on a work surface of a processing apparatus.
Difficulty has been encountered when attempting to utilize conventional machine vision systems to align the printing screen used to deposit the conductive pattern on a photocell wafer. Conventional illumination techniques did not provide sufficient contrast between the wafer and the printing screen to enable the vision system to reliably and accurately detect the edges of the wafer. Both the printing screen and the wafer reflected visible light similarly, which adversely affected the ability to distinguish between those objects. An alternative proposal involved backlighting the printing screen, however placing a light source on the remote side of the printing screen from the wafer interfered with the printing process.
As a consequence, it is desirable to develop an alternative machine vision technique for use with automatic registration between a wafer and the printing screen.